The world's largest blue diamond, an extremely rare gem known as "The Oppenheimer Blue", sold for $57.5 million at Christie's Geneva Wednesday night, making it the most expensive diamond ever to be sold at auction.

The sparkling gem sold to a private collector, whose identity Christie's has not yet revealed.

Blue diamonds are exceptionally rare, comprising only around 0.0001% of the world's diamonds, according to Christie's. Of this tiny fraction, only about 1% are classified as Fancy Vivid, helping to explain the Oppenheimer's wallet-melting price tag.

They are second in rarity only to red diamonds, of which there are only around 100 known record stones in the world.

The Oppenheimer's record-breaking sale comes a day after the 15.38 carat "Unique Pink" sold for $31.6 million at a Sotheby's auction in Geneva -- the highest price ever paid for a pink diamond of that category.